In order to effectively lower the cost and improve the yield of a TFT LCD, the method of manufacturing a TFT LCD array substrate has been gradually simplified from a seven or six-mask process at the beginning to a five-mask process that is popular in the art at present. In recent, a four-mask process based on the slit mask (gray tone mask) technology starts emerging in the field of TFT LCD manufacturing, the key point of which is to replace the second mask (active layer mask) and the third mask (source/drain metal layer mask) of the conventional five-mask process with one slit mask. The four-mask process is described in more detail as follows.
First, a gate electrode is formed on a substrate with the first mask, then a gate insulating layer, an active layer, an ohmic contact layer, and a source/drain metal layer are sequentially deposited on the gate electrode.
Then, with a slit mask, a data line, an active area, a source/drain electrode, and a TFT channel pattern are formed through source/drain metal layer wet etching and a multi-step etching (active layer etching, to ashing, to dry etching, to ohmic contact layer etching).
Then, a passivation layer is deposited on the substrate and a via hole is formed in the passivation layer with the third mask.
Finally, a transparent conductive layer is deposited on the substrate and a pixel electrode is formed with the fourth mask.
Compared with the conventional five-mask process, the above four-mask process is mainly advantageous in that the patterns of the active layer and the source/drain metal layer are formed by one-step slit mask process, thus resulting in the reduction of production period and the manufacturing cost. However, due to use of the slit mask, it demands strict manufacturing precision for the mask. Meanwhile, the pixel-related defects such as channel open and bridge make it hard to improve the yield.
Furthermore, in a conventional back-channel etching type TFT, because there is no etching stop layer over the semiconductor layer (the active layer) for protection, it is necessary to form a relatively thick semiconductor layer in order to void fully etching of the semiconductor layer that may occur when the ohmic contact layer is over-etched. In this case, the leakage current of the TFT LCD array in an off-state increases, thus resulting in great difficulty to improve the contrast ratio of the TFT LCD.